We plan to study the mechanism whereby the genes for folate reductase are amplified during the process whereby cultured mouse cells become resistant to methotrexate, as well as during the period when resistance and the number of genes is decreased. Employing cDNA complementary to folate reductase messenger RNA, we shall study the process employing in situ hybridization to metaphase chromosomes, by studying the structure of the folate reductase genes by restriction endonuclease mapping, and by determining the DNA sequences of the folate reductase gene and adjoining DNA sequences such as DNA segments are obtained by recombinant DNA techniques.